Pitch reduced patterns relative to photolithography features

ABSTRACT

Differently-sized features of an integrated circuit are formed by etching a substrate using a mask which is formed by combining two separately formed patterns. Pitch multiplication is used to form the relatively small features of the first pattern and conventional photolithography used to form the relatively large features of the second pattern. Pitch multiplication is accomplished by patterning a photoresist and then etching that pattern into an amorphous carbon layer. Sidewall spacers are then formed on the sidewalls of the amorphous carbon. The amorphous carbon is removed, leaving behind the sidewall spacers, which define the first mask pattern. A bottom anti-reflective coating (BARC) is then deposited around the spacers to form a planar surface and a photoresist layer is formed over the BARC. The photoresist is next patterned by conventional photolithography to form the second pattern, which is then is transferred to the BARC. The combined pattern made out by the first pattern and the second pattern is transferred to an underlying amorphous silicon layer and the pattern is subjected to a carbon strip to remove BARC and photoresist material. The combined pattern is then transferred to the silicon oxide layer and then to an amorphous carbon mask layer. The combined mask pattern, having features of difference sizes, is then etched into the underlying substrate through the amorphous carbon hard mask layer.

PRIORITY CLAIM

This application is a divisional of U.S. patent application Ser. No.11/214,544, filed Aug. 29, 2005, which claims the priority benefit under35 U.S.C. §119(e) of U.S. Provisional Application No. 60/662,323, filedMar. 15, 2005.

REFERENCE TO RELATED APPLICATIONS

This application is related to and incorporates the following byreference in their entireties: U.S. patent application Ser. No.10/931,772 to Abatchev et al., filed Aug. 31, 2004; U.S. patentapplication Ser. No. 10/932,993 to Abatchev et al., filed Sep. 1, 2004;U.S. patent application Ser. No. 10/931,771 to Tran et al., filed Aug.31, 2004 (now U.S. Pat. No. 7,151,040); U.S. patent application Ser. No.10/934,317 to Sandhu et al., filed Sep. 2, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to integrated circuit fabrication and,more particularly, to masking techniques.

2. Description of the Related Art

As a consequence of many factors, including demand for increasedportability, computing power, memory capacity and energy efficiency,integrated circuits are continuously being reduced in size. The sizes ofthe constituent features that form the integrated circuits, e.g.,electrical devices and interconnect lines, are also constantly beingdecreased to facilitate this size reduction.

The trend of decreasing feature size is evident, for example, in memorycircuits or devices such as dynamic random access memories (DRAMs),flash memory, static random access memories (SRAMs), ferroelectric (FE)memories, etc. To take one example, DRAM typically comprises millions ofidentical circuit elements, known as memory cells. In its most generalform, a memory cell typically consists of two electrical devices: astorage capacitor and an access field effect transistor. Each memorycell is an addressable location that can store one bit (binary digit) ofdata. A bit can be written to a cell through the transistor and can beread by sensing charge in the capacitor. By decreasing the sizes of theelectrical devices that constitute a memory cell and the sizes of theconducting lines that access the memory cells, the memory devices can bemade smaller. Additionally, storage capacities can be increased byfitting more memory cells on a given area in the memory devices.

The continual reduction in feature sizes places ever greater demands onthe techniques used to form the features. For example, photolithographyis commonly used to pattern features, such as conductive lines. Theconcept of pitch can be used to describe the sizes of these features.Pitch is defined as the distance between an identical point in twoneighboring features. These features are typically defined by spacesbetween adjacent features, which spaces are typically filled by amaterial, such as an insulator. As a result, pitch can be viewed as thesum of the width of a feature and of the width of the space on one sideof the feature separating that feature from a neighboring feature.However, due to factors such as optics and light or radiationwavelength, photolithography techniques each have a minimum pitch belowwhich a particular photolithographic technique cannot reliably formfeatures. Thus, the minimum pitch of a photolithographic technique is anobstacle to continued feature size reduction.

“Pitch doubling” or “pitch multiplication” is one proposed method forextending the capabilities of photolithographic techniques beyond theirminimum pitch. A pitch multiplication method is illustrated in FIGS.1A-1F and described in U.S. Pat. No. 5,328,810, issued to Lowrey et al.,the entire disclosure of which is incorporated herein by reference. Withreference to FIG. 1A, a pattern of lines 10 is photolithographicallyformed in a photoresist layer, which overlies a layer 20 of anexpendable material, which in turn overlies a substrate 30. As shown inFIG. 1B, the pattern is then transferred using an etch (preferably ananisotropic etch) to the layer 20, thereby forming placeholders, ormandrels, 40. The photoresist lines 10 can be stripped and the mandrels40 can be isotropically etched to increase the distance betweenneighboring mandrels 40, as shown in FIG. 1C. A layer 50 of spacermaterial is subsequently deposited over the mandrels 40, as shown inFIG. 1D. Spacers 60, i.e., the material extending or originally formedextending from sidewalls of another material, are then formed on thesides of the mandrels 40. The spacer formation is accomplished bypreferentially etching the spacer material from the horizontal surfaces70 and 80 in a directional spacer etch, as shown in FIG. 1E. Theremaining mandrels 40 are then removed, leaving behind only the spacers60, which together act as a mask for patterning, as shown in FIG. 1F.Thus, where a given pitch previously included a pattern defining onefeature and one space, the same width now includes two features and twospaces, with the spaces defined by, e.g., the spacers 60. As a result,the smallest feature size possible with a photolithographic technique iseffectively decreased.

While the pitch is actually halved in the example above, this reductionin pitch is conventionally referred to as pitch “doubling,” or, moregenerally, pitch “multiplication.” Thus, conventionally,“multiplication” of pitch by a certain factor actually involves reducingthe pitch by that factor. The conventional terminology is retainedherein.

Because the layer 50 of spacer material typically has a single thickness90 (see FIGS. 1D and 1E) and because the sizes of the features formed bythe spacers 60 usually correspond to that thickness 90, pitch doublingtypically produces features of only one width. Circuits, however,generally employ features of different sizes. For example, random accessmemory circuits typically contain arrays of memory cells located in onepart of the circuits and logic circuits located in the so-called“periphery.” In the arrays, the memory cells are typically connected byconductive lines and, in the periphery, the conductive lines typicallycontact landing pads for connecting arrays to logic. Peripheral featuressuch as landing pads, however, can be larger than the conductive lines.In addition, periphery electrical devices, including peripheraltransistors, can be larger than the electrical devices in the array.Moreover, even if peripheral features can be formed with the same pitchas features in the array, because mask patterns formed by pitchmultiplication may be limited to those that are formed along thesidewalls of patterned photoresist, pitch multiplication by itselftypically does not offer the flexibility, e.g., geometric flexibility,required to define some features.

To overcome such limitations, some proposed methods for forming patternsat the periphery and in the array involve separately etching patternsinto the array region and the periphery regions of a substrate. Apattern in the array is first formed and transferred to the substrateusing one mask and then another pattern in the periphery is formed andseparately transferred to the substrate using another mask. Because suchmethods form patterns using different masks at different locations on asubstrate, they are limited in their ability to form features thatrequire overlapping patterns, such as when a landing pad overlaps aninterconnect line. As a result, yet a third mask may be necessary to“stitch” two separate patterns of features together. Undesirably, such athird mask would add to the expense and complexity of a process flow andwould face technical challenges in aligning a mask with both the finefeatures defined by the pitch multiplication technique and the typicallylarger peripheral features.

Accordingly, there is a need for methods of forming features ofdifferent sizes, especially where some features are formed below theminimum pitch of a photolithographic technique, and especially inconjunction with pitch multiplication.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a method is provided forsemiconductor fabrication. The method comprises forming an amorphouscarbon layer over a substrate. A lower hard mask layer is formed overthe amorphous carbon layer. An upper hard mask layer is formed on thelower hard mask layer. A temporary layer is formed over the upper hardmask layer. A first hard mask layer is formed over the temporary layer.

According to another aspect of the invention, a method is provided forsemiconductor processing. The method comprises providing a substratehaving an overlying primary mask layer. A hard mask layer formed of afirst material overlies the primary mask layer, a hard mask layer formedof a second material overlies the hard mask layer formed of the firstmaterial, and a pattern comprising pitch-multiplied spacers overlies thehard mask layer comprising the second material. The pattern istransferred to the hard mask layer comprising the second material. Thepattern is subsequently transferred to the hard mask layer formed of thefirst material. The pattern is then transferred to the primary masklayer.

According to yet another aspect of the invention, a method is providedfor semiconductor fabrication. The method comprises forming a firstpattern by pitch multiplication and separately defining a second patternusing photolithography without pitch multiplication. The first andsecond patterns are simultaneously transferred to a hard mask layer. Thefirst and second patterns are then simultaneously transferred from thehard mask layer to an other hard mask layer. The first and secondpatterns are simultaneously transferred from the other hard mask layerto a primary mask layer. The substrate is processed through the primarymask layer.

According to another aspect of the invention, a method is provided forforming a memory device. The method comprises forming a patterncomprising pitch multiplied spacers over a hard mask layer overlying anamorphous carbon layer. The pattern is etched into the hard mask layer.The spacers are subjected to a carbon etch after etching the pattern.The spacers are removed and the pattern is subsequently transferred fromthe hard mask layer to the amorphous carbon layer.

According to yet another aspect of the invention, a method is providedfor integrated circuit fabrication. The method comprises forming anamorphous carbon layer over a substrate and depositing a hard mask layeron the amorphous carbon layer at a temperature less than about 450° C.

According to another aspect of the invention, a partially formedintegrated circuit is provided. The partially formed integrated circuitcomprises a substrate and a primary mask layer overlying the substrate.The primary mask layer is formed of a material different fromphotoresist. A lower hard mask layer overlies the primary mask layer andan upper hard mask layer overlies the lower mask layer. A mask material,which is different from photoresist, defines a first pattern in a firstplane overlying the upper hard mask layer. A photodefinable materialdefines a second pattern over the upper hard mask layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the Detailed Description ofthe Preferred Embodiments and from the appended drawings, which aremeant to illustrate and not to limit the invention, and wherein:

FIGS. 1A-1F are schematic, cross-sectional side views of a sequence ofmasking patterns for forming conductive lines, in accordance with aprior art pitch doubling method;

FIG. 2A is a schematic top plan view of a partially formed integratedcircuit, in accordance with preferred embodiments of the invention;

FIGS. 2B-2C are schematic cross-sectional side views of the partiallyformed integrated circuit of FIG. 2A, in accordance with preferredembodiments of the invention;

FIGS. 3A and 3B are schematic cross-sectional side and top plan views ofthe partially formed integrated circuit of FIG. 2 after forming lines ina photoresist layer in the array region of the integrated circuit, inaccordance with preferred embodiments of the invention;

FIGS. 4A and 4B are schematic cross-sectional side and top plan views ofthe partially formed integrated circuit of FIGS. 3A and 3B afterwidening spaces between lines in the photoresist layer, in accordancewith preferred embodiments of the invention;

FIG. 5 is a schematic, cross-sectional side view of the partially formedintegrated circuit of FIGS. 4A and 4B after etching through a first hardmask layer, in accordance with preferred embodiments of the invention;

FIG. 6 is a schematic, cross-sectional side view of the partially formedintegrated circuit of FIG. 5 after transferring a pattern from the hardmask layer to a temporary layer, in accordance with preferredembodiments of the invention;

FIG. 7 is a schematic, cross-sectional side view of the partially formedintegrated circuit of FIG. 6 after a hard mask layer removal, inaccordance with preferred embodiments of the invention;

FIG. 8 is a schematic, cross-sectional side view of the partially formedintegrated circuit of FIG. 7 after depositing a layer of a spacermaterial, in accordance with preferred embodiments of the invention;

FIGS. 9A and 9B are schematic, cross-sectional side and top plan viewsof the partially formed integrated circuit of FIG. 8 after a spaceretch, in accordance with preferred embodiments of the invention;

FIG. 10 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIGS. 9A and 9B after removing a remainingportion of the temporary layer to leave a pattern of spacers in thearray region of the integrated circuit, in accordance with preferredembodiments of the invention;

FIG. 11 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 10 after surrounding the spacers witha removable planarizing material and forming a photoresist layer overthe spacers, in accordance with preferred embodiments of the invention;

FIG. 12 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 11 after forming a pattern in thephotoresist layer in the periphery of the integrated circuit, inaccordance with preferred embodiments of the invention;

FIG. 13 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 12 after transferring the pattern fromthe photoresist layer to the planarizing material at the same level asthe spacers, in accordance with preferred embodiments of the invention;

FIG. 14 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 13 after etching the pattern in theperiphery and the spacer pattern in the array into an underlying hardmask layer, in accordance with preferred embodiments of the invention;

FIG. 15 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 14 after performing a pattern cleanstep to remove the photoresist and patterned planarizing material, inaccordance with preferred embodiments of the invention;

FIG. 16 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 15 after etching the pattern in theperiphery and the spacer pattern in the array into another underlyinghard mask layer, in accordance with preferred embodiments of theinvention;

FIG. 17 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 16 after transferring both the patternin the periphery and the spacer pattern in the array to a primary masklayer, in accordance with preferred embodiments of the invention;

FIG. 18 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 17 after transferring the peripherypattern and the spacer pattern to the underlying substrate, inaccordance with preferred embodiments of the invention;

FIG. 19 is a schematic, cross-sectional side view of the partiallyformed integrated circuit of FIG. 17 after performing a spacer removaland before transferring the pattern into the substrate, in accordancewith other preferred embodiments of the invention;

FIG. 20 is a micrograph, as viewed through a scanning electronmicroscope, of a side cross section of a pattern etched into both thearray and the periphery of a partially formed integrated circuit, formedin accordance with preferred embodiments of the invention; and

FIGS. 21A and 21B are micrographs, as viewed through a scanning electronmicroscope, of a top view of a pattern etched into the array and theperiphery, respectively, of a partially formed integrated circuit,formed in accordance with preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In addition to problems with forming differently sized features, it hasbeen found that pitch doubling techniques can encounter difficulty intransferring spacer patterns to a substrate. In common methods oftransferring patterns, both the spacers and the underlying substrate areexposed to an etchant, which preferentially etches away the substratematerial. The etchants, however, can also wear away the spacers, albeitat a slower rate. Thus, over the course of transferring a pattern to anunderlying material, the etchant can wear away the spacers before thepattern transfer is complete. These difficulties are exacerbated by thetrend towards decreasing feature size, which, for example, increasinglyleads to the need to form trenches which have increasingly higher depthto width ratios. Thus, in conjunction with difficulties in producingstructures having different feature sizes, pattern transfer limitationsmake the application of pitch multiplication principles to integratedcircuit manufacture even more difficult.

In view of these difficulties, preferred embodiments of the inventionallow for improved pattern transfer and for the formation of differentlysized features in conjunction with pitch multiplication. In a firstphase of methods according to the preferred embodiments, an appropriatesequence of layers of materials is formed to allow formation of a maskfor processing a substrate. In a second phase of methods according tothe preferred embodiments, photolithography and pitch multiplication arepreferably used to form a first pattern defined by spacers. Thistypically forms features of one size in one region of the chip, e.g.,the array of a memory chip. In a third phase, photolithography isperformed to form a second pattern in a mask layer formed over or aroundfeatures forming the first pattern. To allow this photolithography,another photoresist layer can be formed around the spacers or, morepreferably, the spacers are surrounded by a planarizing material andphotoresist layer is preferably formed over the planarizing material.The second pattern can completely or partially overlap the firstpattern, or, in some preferred embodiments, can be completely in adifferent region of the chip, e.g., the periphery of the memory chip.

In a fourth phase, both the first and second patterns are transferred toan underlying primary masking layer, which preferably can bepreferentially etched relative to an underlying substrate. Because theprimary masking layer is preferably used to transfer patterns thesubstrate, various precautions are preferably taken to maintain thestructural and chemical integrity of this layer so that the patternsformed in this layer are well-defined.

As such, the pattern transfer is preferably accomplished by transferringthe first and second patterns consecutively to two hard mask layers andthen to the primary masking layer. It has been found that performing anetch through the planarizing layer or the photoresist layer can resultin polymerization of the photoresist material and/or planarizingmaterial. This polymerization can leave deposits around patternfeatures, thereby distorting features of the first and/or secondpatterns. This distortion can be particularly problematic given thesmall pitches for which pitch multiplication is typically used. As aresult, after etching the first and second patterns into an upper hardmask layer, a cleaning step is preferably performed to remove theplanarizing material, photoresist and any polymerized planarizingmaterial or photoresist. Because the planarizing material, thephotoresist and the underlying primary masking layer are preferably allcarbon-based materials, the cleaning can also undesirably etch theprimary masking layer. This is especially a concern where the cleaningis accomplished using an isotropic etch, which can etch the primary masklayer uncontrollably and typically does not form well-defined features.Thus, a lower hard mask layer is preferably used to protect the primarymasking layer during the cleaning step.

Moreover, the lower hard mask layer and, more preferably, both the lowerand upper hard mask are preferably formed by low temperature depositionprocesses, preferably performed at less than about 550° C. and, morepreferably, at less than about 450° C. and, most preferably, at lessthan about 400° C. Processing at these low temperatures advantageouslyaids in maintaining the integrity of the primary masking layer,especially when that layer is formed of amorphous carbon. For example,undesirable ashing can occur if amorphous carbon is exposed to highertemperatures.

Thus, a preferred material for the primary masking layer is amorphouscarbon. Preferred materials for the spacers include silicon, siliconnitride, or silicon oxide. In other embodiments, the materials for thespacers and the primary masking layer can be reversed. The upper hardmask layer is preferably formed of a material that can be deposited atlow temperatures, as discussed above, and is preferentially etchablerelative to the spacers, the lower hard mask layer and any materialother material overlying the upper hard mask layer. The lower hard masklayer is preferably also formed of a material that can be deposited atlow temperatures and is preferentially etchable relative to the primarymasking layer and the upper hard mask layer. The spacers and the lowerhard mask layer can be formed of different materials, but preferably areformed of the same material to simplify processing and processchemistries. For example, in some embodiments, the spacers and the lowerhard mask layer can be formed of an oxide, e.g., silicon oxide, whilethe upper hard mask layer can be formed of, e.g., silicon, or viceversa. The first and second patterns can then be transferred from one orboth hard mask layers to the primary masking layer.

The first and second patterns are then preferably transferred from theprimary masking layer to the underlying substrate in a single step.Thus, patterns for forming differently sized features, some of which arebelow the minimum pitch of the photolithographic technique used forpatterning, can be formed and these patterns can be successfullytransferred to the underlying substrate. Moreover, because the secondpattern is preferably initially formed in a layer substantiallycoextensive with the first pattern, the second pattern can overlap thefirst pattern. As a result, overlapping features of different sizes onboth sides of the photolithographic limit, such as conducting lines andlanding pads or periphery transistors, can advantageously be formed.

Preferably, the primary masking layer is the masking layer that directlyoverlies and, due to etch selectivity, is primarily used as the mask topattern the substrate. In particular, the primary masking layer ispreferably formed of a material that allows good etch selectivityrelative to both the immediately overlying hard mask material and thesubstrate material, thereby allowing: the spacer pattern in the hardmask layer to be effectively transferred to it; the primary maskinglayer to be selectively removed without harming the substrate; and thepattern in it to be effectively transferred to the substrate. In otherembodiments, particularly where the substrate is relatively simple andcan be selectively etched relative to hard mask materials, the first andsecond patterns can be transferred directly to the substrate using ahard mask, e.g., the lower hard mask discussed above.

As noted above, in common methods of transferring patterns, both themask and the underlying substrate are exposed to etchant, which can wearaway a mask before the pattern transfer is complete. These difficultiesare exacerbated where the substrate comprises multiple differentmaterials to be etched. It is due to its excellent etch selectivityrelative to a variety of materials, including oxides, nitrides andsilicon, that the primary masking layer is preferably formed ofamorphous carbon and, more preferably, transparent carbon.

While the primary mask layer is preferably appropriately thick so thatit is not worn away before the pattern transfer is complete, it will beappreciated that the spacers and upper and lower hard mask layerstypically also overlie the primary mask layer when etching a substrate.It has been found, however, that, in cases where the primary mask layeris particularly thick and/or the mask features are very thin, therelatively tall and thin features in the mask may not be structurallystable. As a result, the mask features can deform and may be unstable.Thus, an optional spacer or spacer and hard mask removal can beperformed to straighten and stabilize the profile of the mask featuresbefore transfer of the pattern to the substrate. In other embodiments,one or both hard mask layers can be removed before transfer of thepattern to the substrate.

It will be appreciated that the “substrate” to which patterns aretransferred can include a layer of a single material, a plurality oflayers of different materials, a layer or layers having regions ofdifferent materials or structures in them, etc. These materials caninclude semiconductors, insulators, conductors, or combinations thereof.For example, the substrate can comprise doped polysilicon, an electricaldevice active area, a silicide, or a metal layer, such as a tungsten,aluminum or copper layer, or combinations thereof. In some embodiments,the mask features discussed below can directly correspond to the desiredplacement of conductive features, such as interconnects, in thesubstrate. In other embodiments, the substrate can be an insulator andthe location of mask features can correspond to the desired location ofinsulators, such as in damascene metallization. Examples of structuresformed in the substrate include gate stacks and shallow trench isolationstructures.

In any of the steps described herein, transferring a pattern from anoverlying level to an underlying level involves forming features in theunderlying level that generally correspond to features in the overlyinglevel. For example, the path of lines in the underlying level willgenerally follow the path of lines in the overlying level and thelocation of other features in the underlying level will correspond tothe location of similar features in the overlying level. The preciseshapes and sizes of features can vary from the overlying level to theunderlying level, however. For example, depending upon etch chemistriesand conditions, the sizes of and relative spacings between the featuresforming the transferred pattern can be enlarged or diminished relativeto the pattern on the overlying level, while still resembling the sameinitial “pattern,” as can be seen from the example of shrinking thefirst resist mask in the embodiments described below. Thus, even withsome changes in the dimensions of features, the transferred pattern isstill considered to be the same pattern as the initial pattern. Incontrast, forming spacers around mask features can change the pattern.

Reference will now be made to the Figures, wherein like numerals referto like parts throughout. It will be appreciated that these Figures arenot necessarily drawn to scale.

In a first phase of methods according to the preferred embodiments, asequence of layers of materials is formed that allow formation of a maskfor processing a substrate.

FIG. 2A shows a top view of a portion of an integrated circuit 100.While the preferred embodiments can be used to form any integratedcircuit, they are particularly advantageously applied to form deviceshaving arrays of electrical devices, including memory cell arrays forvolatile and non-volatile memory devices such as DRAM, ROM or flashmemory, including NAND flash memory, or integrated circuits having logicor gate arrays. For example, the logic array can be a field programmablegate array (FPGA) having a core array similar to a memory array and aperiphery with supporting logics. Consequently, the integrated circuit100 can be, e.g., a memory chip or a processor, which can include both alogic array and embedded memory, or any other integrated circuit havinga logic or a gate array.

With continued reference to FIG. 2A, a central region 102, the “array,”is surrounded by a peripheral region 104, the “periphery.” It will beappreciated that, in a fully formed integrated circuit 100, the array102 will typically be densely populated with conducting lines andelectrical devices such as transistors and capacitors. In a memorydevice, the electrical devices form a plurality of memory cells, whichare typically arranged in a regular grid pattern at the intersection ofword lines and bit lines. Desirably, pitch multiplication can be used toform features such as rows/columns of transistors and capacitors in thearray 102, as discussed below. On the other hand, the periphery 104typically comprises features larger than those in the array 102.Conventional photolithography, rather than pitch multiplication, ispreferably used to pattern features, such as logic circuitry, in theperiphery 104, because the geometric complexity of logic circuitslocated in the periphery 104 makes using pitch multiplication difficult,whereas the regular grid typical of array patterns is conducive to pitchmultiplication. In addition, some devices in the periphery requirelarger geometries due to electrical constraints, thereby making pitchmultiplication less advantageous than conventional photolithography forsuch devices. In addition to possible differences in relative scale, itwill be appreciated by the skilled artisan that the relative positions,and the number of periphery 104 and array 102 regions in the integratedcircuit 100 may vary from that depicted.

FIG. 2B shows a cross-sectional side view of the partially formedintegrated circuit 100. Various masking layers 120-160 are preferablyprovided above a substrate 110. The layers 120-160 will be etched toform a mask for patterning the substrate 110, as discussed below.

The materials for the layers 120-160 overlying the substrate 110 arepreferably chosen based upon consideration of the chemistry and processconditions for the various pattern forming and pattern transferringsteps discussed herein. Because the layers between a topmost selectivelydefinable layer 120 and the substrate 110 preferably function totransfer a pattern derived from the selectively definable layer 120 tothe substrate 110, the layers 130-160 between the selectively definablelayer 120 and the substrate 110 are preferably chosen so that they canbe selectively etched relative to other exposed materials. It will beappreciated that a material is considered selectively, orpreferentially, etched when the etch rate for that material is at leastabout 2-3 times greater, preferably at least about 10 times greater,more preferably at least about 20 times greater and, most preferably, atleast about 40 times greater than that for surrounding materials.Because a goal of the layers 120-155 overlying the primary hard masklayer 160 is to allow well-defined patterns to be formed in that layer160, it will be appreciated that one or more of the layers 120-155 canbe omitted or substituted if suitable other materials, chemistriesand/or process conditions are used. For example, the layer 130 can beomitted in some embodiments where the resolution enhancement propertiesof that layer, as discussed below, are not desired.

In the illustrated embodiment, the selectively definable layer 120overlies a first hard mask, or etch stop, layer 130, which overlies atemporary layer 140, which overlies a second (upper) hard mask, or etchstop, layer 150, which overlies a third (lower) hard mask layer 155,which overlies a primary mask layer 160, which overlies the substrate110 to be processed (e.g., etched) through a mask. Preferably, the maskthrough which the substrate 110 is processed is formed in the third hardmask layer 155 or in the primary mask layer 160.

With continued reference to FIG. 2B, the selectively definable layer 120is preferably photodefinable, e.g., formed of a photoresist, includingany photoresist known in the art. For example, the photoresist can beany photoresist compatible with 157 nm, 193 nm, 248 nm or 365 nmwavelength systems, 193 nm wavelength immersion systems, extremeultraviolet systems (including 13.7 nm wavelength systems) or electronbeam lithographic systems. In addition, maskless lithography, ormaskless photolithography, can be used to define the selectivelydefinable layer 120. Examples of preferred photoresist materials includeargon fluoride (ArF) sensitive photoresist, i.e., photoresist suitablefor use with an ArF light source, and krypton fluoride (KrF) sensitivephotoresist, i.e., photoresist suitable for use with a KrF light source.ArF photoresists are preferably used with photolithography systemsutilizing relatively short wavelength light, e.g., 193 nm. KrFphotoresists are preferably used with longer wavelength photolithographysystems, such as 248 nm systems. In other embodiments, the layer 120 andany subsequent resist layers can be formed of a resist that can bepatterned by nano-imprint lithography, e.g., by using a mold ormechanical force to pattern the resist.

The material for the first hard mask layer 130 preferably comprises aninorganic material. Exemplary materials include silicon oxide (SiO₂),silicon or a dielectric anti-reflective coating (DARC), such as asilicon-rich silicon oxynitride. Preferably, the first hard mask layer130 is a dielectric anti-reflective coating (DARC). Using DARCs for thefirst hard mask layer 130 can be particularly advantageous for formingpatterns having pitches near the resolution limits of aphotolithographic technique. The DARCs can enhance resolution byminimizing light reflections, thus increasing the precision with whichphotolithography can define the edges of a pattern.

The temporary layer 140 is preferably formed of amorphous carbon, which,as noted above, offers very high etch selectivity relative to thepreferred hard mask materials. More preferably, the amorphous carbon isa form of amorphous carbon that is highly transparent to light and thatoffers further improvements for photo alignment by being transparent tothe wavelengths of light used for such alignment. Deposition techniquesfor forming such transparent carbon can be found in A. Helmbold, D.Meissner, Thin Solid Films, 283 (1996) 196-203, the entire disclosure ofwhich is incorporated herein by reference.

The combination of materials for the second and third hard mask layers150 and 155 are preferably chosen based upon the material used for thespacers and for the underlying layer 160. As discussed below, the layer160 is preferably formed of amorphous carbon. Exemplary combinations ofthe other materials are listed in the table below:

Exemplary Spacer and Hard Mask Materials Spacer Oxide Nitride AmorphousCarbon material: silicon Hard mask Amorphous Amorphous Oxide/ Amorphousmaterials silicon/ silicon/oxide amorphous silicon/oxide (Second hardoxide or oxide/ silicon or oxide/ mask/Third amorphous amorphous hardmask): silicon silicon

It will be appreciated that the oxide is preferably a form of siliconoxide and the nitride is typically silicon nitride. Where the spacermaterial is carbon, the temporary layer is preferably a material that ispreferentially etchable relative to the carbon. For example, thetemporary layer can be formed of a silicon-containing material.Depending on the selection of appropriate etch chemistries andneighboring materials, examples of other hard mask materials includeamorphous carbon and etchable high-K materials.

In the illustrated embodiment, the second hard mask layer 150 is formedof silicon, e.g., amorphous silicon. The third hard mask layer 155 isformed of a silicon oxide, e.g., a low silane oxide (LSO). The LSO isformed by chemical vapor deposition using a relatively low silane flowand a relatively high N₂O precursor flow. Advantageously, such adeposition can be performed at relatively low temperatures, e.g., lessthan about 550° C. and, more preferably, less than about 400° C., toprevent damage to the underlying primary mask layer 160, when the layer160 is formed of a temperature-sensitive material. It will beappreciated that oxides can typically be etched with greater selectivityrelative to silicon than nitrides. For example, etch chemistries foroxides can remove the oxides at a rate more than 10 times faster thanamorphous silicon, while etch chemistries for nitrides typically onlyremove the nitrides at a rate of about 3 times faster than amorphoussilicon. As a result, both the spacers and the third hard mask layer arepreferably formed of the same material, an oxide, when the second hardmask layer is formed of amorphous silicon.

As noted above, the primary mask layer 160 is preferably formed ofamorphous carbon due to its excellent etch selectivity relative to manymaterials. As noted above, amorphous carbon is particularly advantageousfor transferring patterns to difficult to etch substrates, such as asubstrate 110 comprising multiple materials or multiple layers ofmaterials, or for forming small and high aspect ratio features.

In addition to selecting appropriate materials for the various layers,the thicknesses of the layers 120-160 are preferably chosen dependingupon compatibility with the etch chemistries and process conditionsdescribed herein. As discussed above, when transferring a pattern froman overlying layer to an underlying layer by selectively etching theunderlying layer, materials from both layers are removed to some degree.Thus, the upper layer is preferably thick enough so that it is not wornaway over the course of the pattern transfer.

In the illustrated embodiment, the photodefinable layer 120 ispreferably about 50-300 nm thick and, more preferably, about 200-250 nmthick. It will be appreciated that, in cases where the layer 120 is aphotoresist, this thickness can vary depending upon the wavelength oflight used to pattern the layer 120. A thickness of about 50-300 nmthick and, more preferably, about 200-250 nm thick is particularlyadvantageous for 248 nm wavelength systems.

The first hard mask layer 130 is preferably about 10-40 nm thick and,more preferably, about 15-30 nm thick. The temporary layer 140 ispreferably about 50-200 nm thick and, more preferably, about 80-120 nmthick. The second hard mask layer 150 is preferably about 20-80 nm thickand, more preferably, about 30-50 nm thick and the third hard mask layer155 is preferably about 10-50 nm thick and, more preferably, about 20-30nm thick.

As discussed above, the thickness of the primary mask layer 160 ispreferably chosen based upon the selectivity of the etch chemistry foretching the substrate and based upon the materials and complexity of thesubstrate. Advantageously, it has been found that a thickness ofpreferably about 100-500 nm and, more preferably, about 200-300 nm isparticularly effective for transferring patterns to a variety ofsubstrates, including substrates having a plurality of differentmaterials to be etched during the transfer.

For example, FIG. 2C shows an exemplary substrate 160 comprising aplurality of layers which can be etched to form control gate stacks. Asilicide layer 110 a overlies a polysilicon layer 110 b, which overliesan oxide-nitride-oxide (ONO) composite layer 110 c, which overlies apolysilicon layer 110 d.

The various layers discussed herein can be formed by various methods.For example, spin-on-coating processes can be used to formphotodefinable layers. Various vapor deposition processes, such aschemical vapor deposition, can be used to form hard mask layers.

Preferably, a low temperature chemical vapor deposition (CVD) process isused to deposit the hard mask layers or any other materials, e.g.,spacer material, over the primary mask layer 160, especially in caseswhere the primary mask layer 160 is formed of amorphous carbon.

Advantageously, it has been found that the second and third hard masklayers 150 and 155 can be deposited at temperatures of less than about550° C. and, more preferably, less than about 450° C. and, mostpreferably, less than about 400° C. Such low temperature depositionprocesses advantageously prevent chemical or physical disruption of theamorphous carbon layer(s).

For example, a LSO, e.g., for forming either the layers 150 or 155, canbe deposited by a plasma enhanced CVD (PECVD) process. Variousprocessing systems made by various manufacturers can be used to performthe process, as known in the art. A non-limiting example of a suitablereactor system is the Applied Materials' Producer™ system. In oneexample of process conditions, SiH₄ is preferably flowed into thereactor at a rate of about 50-250 sccm and, more preferably, about 150sccm. N₂₀ is flowed into the reactor at a rate of about 400-1000 sccmand, more preferably, about 750 sccm, and He is flowed into the reactorat a rate of about 2500-4000 sccm and, more preferably, about 3500 sccm.The pressure within reactor is preferably maintained at about 4-8 torrand, more preferably, about 6.5 torr. The RF power is preferably about50-200 watts and, more preferably, about 110 watts. The spacing ispreferably about 400-600 mils and, more preferably, about 450 mils.Advantageously, it has been found that the LSO can be deposited at atemperature of about 250-450° C. and, more preferably, about 375° C.

It has been found that amorphous silicon, e.g., for forming the other ofthe layers 150 or 155 can also be deposited at low temperatures by aplasma enhanced CVD (PECVD) process. In one example, SiH₄ and He aredelivered to the reactor in an Applied Materials' Producer™ system. TheSiH₄ is preferably flowed at about 80-300 sccm and, more preferably,about 150 sccm. The He is flowed at about 400-300 sccm and, morepreferably, about 1800 sccm. The pressure within the reactor ispreferably about 3-5 torr and, more preferably, about 3.5 torr and theRF power is preferably about 50-200 watts and, more preferably, about100 watts. The spacing is preferably about 400-600 mils and, morepreferably, about 450 mils. Advantageously, the amorphous silicon can bedeposited at a temperature of about 250-450° C., and, more preferably,about 375° C.

In addition, the amorphous carbon layers can be formed by chemical vapordeposition using a hydrocarbon compound, or mixtures of such compounds,as carbon precursors. Exemplary precursors include propylene, propyne,propane, butane, butylene, butadiene and acetelyne. A suitable methodfor forming amorphous carbon layers is described in U.S. Pat. No.6,573,030 B1, issued to Fairbaim et al. on Jun. 3, 2003, the entiredisclosure of which is incorporated herein by reference. In addition,the amorphous carbon may be doped. A suitable method for forming dopedamorphous carbon is described in U.S. patent application Ser. No.10/652,174 to Yin et al., the entire disclosure of which is incorporatedherein by reference.

In a second phase of methods according to the preferred embodiments, apattern of spacers is formed by pitch multiplication.

With reference to FIGS. 3A and 3B, a pattern comprising spaces ortrenches 122, which are delimited by photodefinable material features124, is formed in the photodefinable layer 120. The trenches 122 can beformed by, e.g., photolithography with 248 nm or 193 nm light, in whichthe layer 120 is exposed to radiation through a reticle and thendeveloped. After being developed, the remaining photodefinable material,photoresist in the illustrated embodiment, forms mask features such asthe illustrated lines 124 (shown in cross-section only).

The pitch of the resulting lines 124 is equal to the sum of the width ofa line 124 and the width of a neighboring space 122. To minimize thecritical dimensions of features formed using this pattern of lines 124and spaces 122, the pitch can be at or near the limits of thephotolithographic technique used to pattern the photodefinable layer120. For example, for photolithography utilizing 248 nm light, the pitchof the lines 124 can be about 100 nm. Thus, the pitch may be at theminimum pitch of the photolithographic technique and the spacer patterndiscussed below can advantageously have a pitch below the minimum pitchof the photolithographic technique. Alternatively, because the margin oferror for position and feature size typically increases as the limits ofa photolithographic technique are approached, the lines 124 can beformed having larger feature sizes, e.g., 200 nm, to minimize errors inthe position and sizes of the lines 124.

As shown in FIGS. 4A and 4B, the spaces 122 are preferably widened byetching the photoresist lines 124, to form modified spaces 122 a andlines 124 a. The photoresist lines 124 are preferably etched using anisotropic etch to “shrink” those features. Suitable etches includeetches using an oxygen-containing plasma, e.g., a SO₂/O₂/N₂/Ar plasma, aCl₂/O₂/He plasma or a HBr/O₂/N₂ plasma. The extent of the etch ispreferably selected so that the widths of the lines 124 a aresubstantially equal to the desired spacing between the later-formedspacers 175, as will be appreciated from the discussion below. Forexample, the width of the lines 124 can be reduced to from about 80-120nm to about 40-70 nm. Advantageously, the width-reducing etch allows thelines 124 a to be narrower than would otherwise be possible using thephotolithographic technique used to pattern the photodefinable layer120. In addition, the etch can smooth the edges of the lines 124 a, thusimproving the uniformity of those lines. While the critical dimensionsof the lines 124 a can be etched below the resolution limits of thephotolithographic technique, it will be appreciated that this etch doesnot alter the pitch of the spaces 122 a and lines 124 a, since thedistance between identical points in these features remains the same.

With reference to FIG. 5, the pattern in the (modified) photodefinablelayer 120 a is transferred to the hard mask layer 130. This transfer ispreferably accomplished using an anisotropic etch, such as an etch usinga fluorocarbon plasma, although a wet (isotropic) etch may also besuitable if the hard mask layer 130 is thin. Preferred fluorocarbonplasma etch chemistries include CFH₃, CF₂H₂, CF₃H and CF₄/HBr.

With reference to FIG. 6, the pattern in the photodefinable layer 120 aand the hard mask layer 130 is transferred to the temporary layer 140 toallow for deposition of a layer 170 of spacer material (FIG. 8). It hasbeen found that the temperatures used for spacer material deposition aretypically too high for photoresist to withstand. Thus, the pattern ispreferably transferred from the photodefinable layer 120 a to thetemporary layer 140, which is formed of a material that can withstandthe process conditions for spacer material deposition and etch,discussed below. In addition to having higher heat resistance thanphotoresist, the material forming the temporary layer 140 is preferablyselected such that it can be selectively removed relative to thematerial for the spacers 175 (FIG. 10) and the underlying etch stoplayer 150. As noted above, the layer 140 is preferably formed ofamorphous carbon and, more preferably, transparent carbon.

The pattern in the modified photodefinable layer 120 a is preferablytransferred to the temporary layer 140 using a O₂-containing plasma,e.g., a plasma containing SO₂, O₂ and Ar. Other suitable etchchemistries include a Cl₂/O₂/SiCl₄ or SiCl₄/O₂/N₂ or HBr/O₂/N₂/SiCl₄containing plasma. Advantageously, the SO₂-containing plasma is used asit can etch carbon of the preferred temporary layer 140 at a rategreater than 20 times and, more preferably, greater than 40 times therate that the hard mask layer 130 is etched. A suitable SO₂-containingplasma is described in U.S. patent application Ser. No. 10/931,772 toAbatchev et al., filed Aug. 31, 2004, the entire disclosure of which isincorporate herein by reference. It will be appreciated that theSO₂-containing plasma can simultaneously etch the temporary layer 140and also remove the photodefinable layer 120 a. The resulting lines 124b constitute the placeholders or mandrels along which a pattern ofspacers 175 (FIG. 10) will be formed.

With reference to FIG. 7, the hard mask layer 130 can be removed tofacilitate later spacer formation by leaving the temporary layer 140exposed for subsequent etching (FIG. 10). The hard mask layer 130 can beremoved using a buffered oxide etch (BOE), which is a wet etchcomprising HF and NH₄F.

Next, as shown in FIG. 8, a layer 170 of spacer material is preferablyblanket deposited conformally over exposed surfaces, including the hardmask layer 150 and the top and sidewalls of the temporary layer 140. Thespacer material can be any material that can act as a mask fortransferring a pattern to the underlying hard mask layer 150. The spacermaterial preferably: 1) can be deposited with good step coverage; 2) canbe deposited at a temperature compatible with the temporary layer 140;and 3) can be selectively etched relative to the temporary layer 140 andunderlying hard mask layer 150. Preferred materials include silicon,silicon oxides and silicon nitrides. In the illustrated embodiment, thespacer material is silicon oxide, which provides particular advantagesin combination with other selected materials of the masking stack.

Preferred methods for spacer material deposition include chemical vapordeposition, e.g., using O₃ and TEOS to form silicon oxide, and atomiclayer deposition, e.g., using a silicon precursor with an oxygen ornitrogen precursor to form silicon oxides and nitrides, respectively.The thickness of the layer 170 is preferably determined based upon thedesired width of the spacers 175 (FIG. 10). For example, in the oneexemplary embodiment, the layer 170 is preferably deposited to athickness of about 20-80 nm and, more preferably, about 40-60 nm.Preferably, the step coverage is about 80% or greater and, morepreferably, about 90% or greater.

With reference to FIGS. 9A and 9B, the silicon oxide spacer layer 170 isthen subjected to an anisotropic etch to remove spacer material fromhorizontal surfaces 180 of the partially formed integrated circuit 100.Such an etch, also known as a spacer etch, can be performed using afluorocarbon plasma, e.g., containing CF₄/CHF₃, C₄F₈/CH₂F₂ or CHF₃/Arplasma.

With reference to FIG. 10, the temporary layer 140 is next removed toleave freestanding spacers 175. The temporary layer 140 is selectivelyremoved using an organic strip process. Preferred etch chemistriesinclude a oxygen-containing plasma etch, such as an etch using SO₂.

Thus, pitch multiplication has been accomplished. In the illustratedembodiment, the pitch of the spacers 175 is roughly half that of thephotoresist lines 124 and spaces 122 (FIG. 3A) originally formed byphotolithography. Where the photoresist lines 124 had a pitch of about200 nm, spacers 175 having a pitch of about 100 nm or less can beformed. It will be appreciated that because the spacers 175 are formedon the sidewalls of the features or lines 124 b, the spacers 175generally follow the outline of the pattern of features or lines 124 ain the modified photodefinable layer 120 a and, so, typically form aclosed loop in the spaces 122 a between the lines 124 a. The spacers 175form a first pattern 177.

Next, in a third phase of methods according to the preferredembodiments, a second pattern is formed over the first pattern 177.Preferably, the second pattern comprises features having larger criticaldimensions than the first pattern 177. In addition, the second patterncan be formed completely, partially, or not overlapping the firstpattern 177.

To allow the second pattern to be formed, a planar surface is formed bydepositing a planarizing material around the spacers 175 to form aplanarizing layer 200, as shown in FIG. 11. A selectively definablelayer 220 is then formed on the planarizing material to allow forpatterning of the second pattern at the periphery 104.

The planarizing layer 200 is preferably at least as tall as the spacers175. In addition, the protective layer 200 is preferably formed of amaterial that can be selectively etched relative to both the spacers 175and the selectively definable layer 220. For example, the planarizinglayer 200 can be formed of a spin-on anti-reflective coating, such as abottom anti-reflective coating (BARC).

As with the selectively definable layer 120, the selectively definablelayer 220 is preferably photodefinable, e.g., formed of a photoresist,including any photoresist known in the art. In addition, in otherembodiments, the layer 220 can be formed of a resist suitable forpatterning by nano-imprint lithography.

In some preferred embodiments, the planarizing layer 200 can be omittedand the selectively definable layer 220 can be formed directly on andaround the spacers 175. Such a scheme can be employed where the patternscan be defined in the layer 220 with good integrity and where theresolution enhancement properties of an anti-reflective coating are notdesired. For example, the anti-reflective coating can be omitted if thematerial underlying the selectively definable layer 220 is sufficientlynon-reflective.

With reference to FIG. 12, the photodefinable layer 220 is patternedusing, e.g., the same photolithographic technique used to pattern thephotodefinable layer 120. Thus, a pattern 230 is formed in thephotodefinable layer 220. Where the pattern 230 is used to mask featuresin the periphery 104, the area in the photodefinable layer 220 in thearray 102 is preferably open, as illustrated. As noted above, however,while illustrated laterally adjacent the pattern 177, the pattern 230can partially or completely overlap the pattern 177 or be completelyseparated from the pattern 177. Thus, the use of different referencenumerals (177 and 230) for these patterns indicates that they wereoriginally formed in different steps.

While the pattern 177 preferably has a pitch or feature size smallerthan the minimum pitch or resolution of the photolithographic techniqueused in forming it, the pattern 230 preferably has a pitch or featuresize equal to or greater than the minimum pitch or resolution of thephotolithographic technique used to form that pattern. It will beappreciated that the pattern 230 at the periphery 104 can be used toform landing pads, transistors, local interconnects, etc.

In a fourth phase of methods according to the preferred embodiments, thepatterns 177 and 230 are consolidated on one level below the spacers andsimultaneously transferred to the substrate 110.

With reference to FIG. 13, the pattern 230 is transferred to the samelevel as the pattern 177 of spacers 175. An anisotropic BARC etch isperformed to define the periphery features in the protective layer 210and to also open up the array features. The parts of the protectivelayer 210 that are unprotected by parts of the photodefinable layer 220are preferably selectively etched using an anisotropic etch, using,e.g., a HBr/O₂ plasma or a SO₂-containing plasma. This etchpreferentially removes the protective layer 200 around the oxide spacers175, thereby leaving those spacers 175 exposed.

With reference to FIGS. 14-16, the second and third hard mask layers areetched to transfer the patterns 177 and 230 down to the primary masklayer 160, to form a mixed pattern in the primary mask layer 160. Withreference to FIG. 14, the patterns 177 and 230 are first bothtransferred to the second hard mask layer 150. Where the second hardmask 150 is formed of amorphous silicon, it is preferablyanisotropically etched using, e.g., a HBr and Cl₂ containing plasma.Such an etch preferably etches the amorphous silicon at a rate greaterthan about 5 times and, more preferably, greater than about 10 times therate at which the silicon oxide spacers 175 and silicon oxide third hardmask 155 can be etched.

With reference to FIG. 15, the first and second patterns 177 and 230 arecleaned. As noted above, the carbon material forming the photoresist andDARC layers 220 and 210 can polymerize upon contact with etchants. Forexample, the HBr/Cl₂ etch of the second hard mask layer 150 can causeparts of the layers 220 and 210 to polymerize and leave a residue aroundfeatures in the second hard mask layer 150, causing a pattern havingundesirably non-uniform features. Thus, the patterns 177 and 230 arepreferably cleaned by stripping off an organic material. The strip canbe accomplished using, e.g., an isotropic etch with O₂ plasma.

With reference to FIG. 16, the patterns 177 and 230 are then bothtransferred to the third hard mask layer 155. Where the third hard mask155 is formed of a LSO, it is preferably anisotropically etched using,e.g., a fluorocarbon plasma. The fluorocarbon plasma preferably includesC₄F₈, CH₂F₂, Ar and O₂ and can preferably etch the silicon oxide and theamorphous carbon at equal rates and, more preferably, can etch thesilicon oxide at a rate greater than about 10 times the rate at whichthe amorphous silicon layer 150 is etched.

With reference to FIG. 17, the patterns 177 and 230 are transferred tothe primary mask layer 160. The transfer is preferably accomplished byanisotropically etching the primary mask layer 160, preferably using aSO₂-containing plasma. Other suitable etch chemistries include a Cl₂/O₂,HBr/O₂/N₂ or SiCl₄/O₂/N₂/HBr or SiCl₄/O₂-containing plasma. As notedabove, the SO₂-containing plasma is preferably used as it has been foundto have excellent selectivity for the amorphous carbon of the primarymask layer 160 relative to the hard mask layers 150 and 155. Thus, athick enough mask can be formed in the primary mask layer 160 to latereffectively transfer the mask pattern to the substrate 110, particularlythrough multiple materials of the substrate using selective etchchemistries and without wearing away the primary mask layer 160 beforethe pattern transfer is complete.

With reference to FIG. 18, after being transferred to the primary masklayer 160, the patterns 177 and 230 are transferred to the substrate 110using the layer 160 as a mask. Given the disparate materials typicallyused for the primary mask layer 160 and the substrate 110 (e.g.,amorphous carbon and silicon or silicon compounds, respectively), thepattern transfer can be readily accomplished using etch chemistriesappropriate for etching the material or materials of the substrate 110.For example, a fluorocarbon etch comprising CF₄, CHF₃ and/or NF₃containing plasma can be used to etch silicon nitride, a fluorocarbonetch comprising CF₄, CHF₃, CH₂F₂ and/or C₄F₈ containing plasma can beused to etch silicon oxide and a HBr, Cl₂, NF₃, SF₆ and/or CF₄containing plasma etch can be used to etch silicon. In addition, theskilled artisan can readily determine suitable etch chemistries forother substrate materials, such as conductors, including aluminum,transition metals, and transition metal nitrides. For example, analuminum substrate can be etched using a fluorocarbon etch.

It will be appreciated that where the substrate 110 comprises layers ofdifferent materials, a succession of different chemistries, preferablydry-etch chemistries, can be used to successively etch through thesedifferent layers, if a single chemistry is not sufficient to etch allthe different materials. It will also be appreciated that, dependingupon the chemistry or chemistries used, the spacers 175 and the hardmask layer 150 may be etched. Using amorphous carbon for the primarymask layer 160, however, advantageously offers excellent resistance toconventional etch chemistries, especially those used for etchingsilicon-containing materials. Thus, the primary mask layer 160 caneffectively be used as a mask for etching through a plurality ofsubstrate layers, or for forming high aspect ratio trenches. Inaddition, the pitch doubled pattern 177 and the pattern 230 formed byconventional lithography can simultaneously be transferred to thesubstrate 110, or each individual layer of the substrate 110, in asingle etch step.

In one example, the sequence of substrate layers 110 a-110 d can beetched using various etch chemistries, which preferably anisotropicallyetch the various layers. The silicide layer 110 a can be etched using aCl₂/CF₄ plasma at a pressure of about 3-10 mTorr, with about 200-350watt source power and about 50-100 watt bias power; the polysiliconlayer 110 b can etched be using a HBr/Cl₂ plasma at a pressure of about10-30 mTorr, with about 300-500 watt source power and about 20-50 wattbias power; the oxide-nitride-oxide (ONO) composite layer 110 c can beetched using a CF₄/CH₂F₂/He plasma at a pressure of about 5-10 mTorr,with about 600-1000 watt source power and about 200-400 watt bias power;and the polysilicon layer 110 d can be etched using a HBr/He/O₂ plasmaat a pressure of about 40-80 mTorr, with about 250-400 watt source powerand about 50-100 watt bias power.

With reference to FIG. 19, in some preferred embodiments, the spacers175 can be removed before using the primary mask layer 160 to transferthe patterns 177 and 230 to the substrate 110. The removal is preferablyperformed using an etch selective for the spacers 175. For example,where the spacers 175 comprise a silicon oxide, the spacer removal canbe accomplished using a wet or dry etch, e.g., a wet buffered oxide etchor a dry etch using a CH₂F₂/C₄F₈/Ar/O₂ plasma. As noted above, thisspacer removal can advantageously straighten and/or stabilize theprofile of the features forming the patterns 177 and 230, especiallywhere the features are taller than optimal for etching the substrate110.

FIG. 20 shows a structure resulting after etching the substrate 110. Asnoted above, the substrate 110 can be any layer of material or materialsthat the patterns 177 and 230 are etched into. The composition of thesubstrate 110 can depend upon, e.g., the electrical device to be formed.Thus, in FIG. 19, the substrate 110 comprises a silicide layer 110 a, apolysilicon layer 110 b, an oxide-nitride-oxide (ONO) composite layer110 c and a floating gate (FG) polysilicon layer 110 d. On the righthand side of the figure, this sequence of layers forms a source selectgate (SG) control line 110 e. Note that all the illustrated features arelocated in the array, although the SG control line 110 e has arelatively large critical dimension due to being defined using thepattern 230. Such an arrangement of layers can be advantageously used inthe formation of, e.g., a control gate stack for NAND flash memory.

Note that the etched surfaces exhibit exceptionally low edge roughness.In addition, the trenches formed in the array show excellent uniformity,even at the low 100 nm pitch (50 nm feature size) pictured.Advantageously, these results are achieved while also formingwell-defined and smooth lines in the periphery, which can have a widthsignificantly greater than about 100 nm, e.g., about 250 nm in theillustrated structure.

It will be appreciated that the formation of patterns according to thepreferred embodiments offers numerous advantages. For example, theability to deposit the second and third hard mask layers 150 and 155 atlow temperatures of, e.g., less than about 550° C., more preferably,less than about 400° C. maintains the structural and chemical integrityof the amorphous carbon layer 160. Moreover, the third hard mask layer155 can provide a buffer to protect the amorphous carbon layer 160 frometch chemistries employed for overlying materials. Advantageously, thethird hard mask layer 155 allows overlying patterns to be cleanedwithout undesirably etching the amorphous carbon layer 160. Thus, thedefinition of the patterns can be improved and unwanted materials, suchas polymerized organics, can be effectively removed.

In addition, because multiple patterns, with differently-sized features,can be consolidated on a single final mask layer before beingtransferred to a substrate, overlapping patterns can easily betransferred to the substrate. Thus, pitch-doubled features and featuresformed by conventional photolithography can easily be formed connectedto each other. Moreover, as evident in FIG. 20, exceptionally smallfeatures can be formed, while at the same time achieving exceptionallylow line edge roughness. While not limited by theory, it is believedthat such low line edge roughness is the result of the use of the layers140 and 160. Forming the spacers 175 and performing multiple anisotropicetches to transfer the patterns 177 and 230 from the level of thetemporary layer 140 to the primary mask layer 160 and then to thesubstrate 110 are believed to beneficially smooth the surfaces of thefeatures forming the patterns 177 and 230. Moreover, the preferredamorphous carbon etch chemistries disclosed herein allow the use of thinhard mask layers, such as the layers 130, 150, and 155 relative to thedepth that underlying amorphous carbon layers, such as the layers 140and 160, are etched. This advantageously allows the layers 140 and 160to be more easily and effectively etched. In addition, demands on theidentity and etch selectivity for the layers (e.g., the photoresistlayers in FIG. 5) overlying the hard mask layers are reduced, since thehard mask layers 130, 150 and 155 do not need to be etched to a greatdepth.

It will also be appreciated that various modifications of theillustrated process flow are possible. For example, pitch multipliedpatterns typically formed closed loops, since the patterns are formed byspacers that formed along the wall of a mandrel. Consequently, where thepitch multiplied pattern is used to form conductive lines, additionalprocessing steps are preferably used to cut off the ends of these loops,so that each loop forms two individual, non-connected lines. This can beaccomplished, for example, by forming a protective mask around the partsof the lines to be maintained, while etching away the unprotected endsof the masks. A suitable method for cutting off the ends of the loops isdisclosed in U.S. patent application Ser. No. 10/931,771 to Tran et al.,filed Aug. 31, 2004, the entire disclosure of which is incorporated bereference herein.

In addition to forming gate control stacks, it will be appreciated thatthe preferred embodiments can be employed to form interconnect lines andassociated integrated circuit features, such as landing pads. FIGS. 21Aand 21B show top views of an integrated circuit after the etching waythe ends of the loops to form individual conductive interconnects. FIG.21A shows the ends of the loops formed with landing pads for eachinterconnect, while FIG. 21B shows the other end of the interconnects.It will be appreciated that the magnifications for each figure isdifferent. Methods for forming interconnects and landing pads aredisclosed in U.S. patent application Ser. No. 10/931,771 to Tran et al.,filed Aug. 31, 2004, the entire disclosure of which is incorporatedherein by reference.

It will also be appreciated that the pitch of the pattern 177 can bemore than doubled. For example, the pattern 177 can be further pitchmultiplied by forming spacers around the spacers 175, then removing thespacers 175, then forming spacers around the spacers that were formerlyaround the spacers 175, and so on. An exemplary method for further pitchmultiplication is discussed in U.S. Pat. No. 5,328,810 to Lowrey et al.In addition, while the preferred embodiments can advantageously beapplied to form patterns having both pitch multiplied and conventionallyphotolithographically defined features, the patterns 177 and 230 canboth be pitch multiplied or can have different degrees of pitchmultiplication.

Moreover, more than two patterns 177 and 230 can be consolidated on theprimary mask layer 160 if desired. In such cases, additional mask layerscan be deposited between the layers 140 and 160. For example, thepatterns 177 and 230 can be transferred to an additional mask layeroverlying the hard mask layer 150 and then the sequence of stepsillustrated in FIGS. 11-16 can be performed to protect the patterns 177and 230, to form a new pattern in an overlying photodefinable layer, andto transfer the patterns to the substrate 110. The additional mask layerpreferably comprises a material that can be selectively etched relativeto the hard mask layer 150 and a protective layer that surrounds thepatterns 177 and 230 after being transferred to the additional masklayer.

In addition, the preferred embodiments can be employed multiple timesthroughout an integrated circuit fabrication process to form pitchmultiplied features in a plurality of layers or vertical levels, whichmay be vertically contiguous or non-contiguous and vertically separated.In such cases, each of the individual levels to be patterned wouldconstitute a substrate 110 and the various layers 120-220 can formedover the individual level to be patterned. It will also be appreciatedthat the particular composition and height of the various layers 120-220discussed above can be varied depending upon a particular application.For example, the thickness of the layer 160 can be varied depending uponthe identity of the substrate 110, e.g., the chemical composition of thesubstrate, whether the substrate comprises single or multiple layers ofmaterial, the depth of features to be formed, etc., and the availableetch chemistries. In some cases, one or more layers of the layer 120-220can be omitted or more layers can be added. For example, the layer 160can be omitted in cases where the hard mask layers 150 and/or 155 aresufficient to adequately transfer a pattern to the substrate 110.

Also, while “processing” through the various mask layers preferablyinvolves etching an underlying layer, processing through the mask layerscan involve subjecting layers underlying the mask layers to anysemiconductor fabrication process. For example, processing can involveion implantation, diffusion doping, depositing, or wet etching, etc.through the mask layers and onto underlying layers. In addition, themask layers can be used as a stop or barrier for chemical mechanicalpolishing (CMP) or CMP can be performed on any of the layers to allowfor both planarization and etching of the underlying layers, asdiscussed in U.S. Provisional Patent Application No. 60/666,031, filedMar. 28, 2005, the entire disclosure of which is incorporated byreference herein.

Accordingly, it will be appreciated by those skilled in the art thatvarious other omissions, additions and modifications may be made to themethods and structures described above without departing from the scopeof the invention. All such modifications and changes are intended tofall within the scope of the invention, as defined by the appendedclaims.

1. A method for semiconductor fabrication, comprising: providing anamorphous carbon layer over a substrate; providing a lower hard masklayer over the amorphous carbon layer; providing an upper hard masklayer on the lower hard mask layer, the upper hard mask layer formed ofa different material than the lower hard mask layer; providing a maskwith a mask pattern on the upper hard mask layer; transferring the maskpattern to the upper hard mask layer; transferring a pattern in theupper hard mask layer to the lower hard mask layer in a next patterntransfer step immediately following transferring the mask pattern to theupper hard mask layer; transferring a pattern in the lower hard masklayer to the amorphous carbon layer; and transferring a pattern in theamorphous carbon layer into the substrate.
 2. The method of claim 1,wherein features in the mask disposed on the upper hard mask layer havea pitch of about 100 nm or less.
 3. The method of claim 1, whereintransferring the pattern in the lower hard mask layer to the amorphouscarbon layer comprises performing a plasma etch comprising SO₂.
 4. Themethod of claim 1, wherein the amorphous carbon is transparent carbon.5. The method of claim 1, wherein transferring the pattern to the upperhard mask layer; transferring the pattern in the upper hard mask layerto the lower hard mask layer; transferring the pattern in the lower hardmask layer to the amorphous carbon layer; and transferring the patternin the amorphous carbon layer to the substrate each comprise performingan anisotropic etch.